Conventionally, in at least some pixels constituting an image sensor, each pixel is configured so as to have a plurality of photodiodes (PDs), and a signal is read out separately from each PD. Technology exists that separately reads out signals from a plurality of PDs, the signals being in accordance with light converged by a single microlens provided in each pixel, and performs focus detection by a phase difference detection method using the read signals of the pixels. For example, Japanese Patent Laid-Open No. 2008-193527 discloses technology in which, by configuring a pixel to have two PDs, the respective PDs are configured so as to receive light fluxes that passed through different exit pupil regions of an image sensing lens. According to the aforementioned technology, focus detection of the image sensing lens is performed by comparing signals obtained from two PDs of each of a plurality of pixels.
Further, Japanese Patent Laid-Open No. 2001-250931 discloses a driving method that performs focus detection by comparing signals obtained from two PDs of each of a plurality of pixels, and further, adds together the signals of the two PDs for each pixel to thereby acquire the added signal as a signal for an image. That is, a single driving operation to perform charge accumulation of divided PDs and read out accumulated charge signals serves as an operation for both acquisition of a signal for focus detection and acquisition of a signal for an image. Accordingly, this driving method is effective in the case of a function in which display and recording at high speed are required while maintaining a focused state of an object, such as when performing sequential imaging of still images, live-view imaging, or video recording.
However, when adding signals of a plurality of PDs to obtain a signal for an image, in some cases an appropriate image cannot be obtained if there is a difference in the sensitivity or incident light amount between the PDs. Such a case will now be described using FIG. 13A, FIG. 13B, and FIG. 14.
FIG. 13A is a view showing a cross-sectional configuration of a pixel of a conventional image sensor. FIG. 13B is a view that schematically illustrates the potential of a semiconductor layer. In FIG. 13A, reference numeral 1401 denotes a microlens, reference numeral 1402 denotes a color filter, and reference numeral 1403 denotes wiring for driving switches inside the pixel and wiring of a power source or the like. A p-type semiconductor region 1405 is formed on a semiconductor substrate 1404, and an n-type region is formed in the semiconductor substrate 1404, by which region a PD is formed. In this case, a PD 1406 and a PD 1407 are formed. Reference numeral 1408 denotes a separation area that separates adjacent pixels. Reference numeral 1409 denotes a separation area that separates PDs that are formed in the same pixel (in this case, the separation area separates the PD 1406 and the PD 1407).
In an image sensor having the above described structure, due to a difference in the sensitivity or an incident light amount, when one of the PDs is saturated, an electrical charge generated at the saturated PD flows over a barrier of a potential (Vw), and leaks to the other PD in the same pixel or to a PD of an adjacent pixel. Furthermore, in some cases the charge may flow over a potential barrier below a gate electrode of an unshown transfer switch that is present between the PD and a region for reading out a charge of the PD, and also leak to the region for reading out a charge of the PD. Conventionally, since the size of the potential (height of the potential barrier) of the separation areas 1408 and 1409 is not taken into consideration, it is assumed that normally a large part of the charge of the saturated PD leaks to a PD of an adjacent pixel or to a region for reading out a charge of the PD. In a case in which the potential of the separation area 1409 is a potential such that a charge generated at the saturated PD does not leak to the other PD in the pixel in this manner, when the signals of the two PDs are added to obtain a signal for an image, the outputs are as shown in FIG. 14.
FIG. 14 shows an example of the output characteristics of the two PDs 1406 and 1407, respectively, and an example of the output characteristic of a signal obtained by adding signals of the two PDs 1406 and 1407. When light is incident on a PD, an electric charge is generated with a certain sensitivity by photoelectric conversion, and the charge is accumulated. In FIG. 14, for the purpose of description, it is assumed that the sensitivity of the PD 1406 is higher than that of the PD 1407, and/or a larger amount of light is incident on the PD 1406. During a period in which the amount of light incident on the PDs 1406 and 1407 is within a range 1501, although the output of the PD 1406 is greater than that of the PD 1407, since the PD 1406 is not saturated, an appropriate output is obtained by adding the outputs of the PD 1406 and the PD 1407. However, when the PD 1406 saturates and the PD 1407 is not saturated, an output that is greater than the saturated amount cannot be obtained as the output of the PD 1406, whereas an appropriate output that is in accordance with the amount of incident light is obtained at the PD 1407. Consequently, from the time point that the PD 1406 saturates, the added output changes in accordance with the output of the PD 1407. As a result, the curve of the added output characteristic has a knee characteristic that bends from the point at which the PD 1406 saturates. This phenomenon noticeably appears when a charge generated after the PD 1406 is saturated leaks to an area other than the PD 1407.